1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography system of the type having gradient coils arranged in a basic field magnet, an RF transmission and reception coil, and a drive and evaluation circuit.
2. Description of the Prior Art
In all conventional magnetic resonance tomography systems, particular value is placed thereon making the magnetic field of the basic field magnet as uniform as possible in the scan volume, i.e. the volume roughly corresponding to a sphere having a diameter of 50 cm within which one strives to obtain an exact image. This volume is also referred to as the DSV (design shim volume). For example, a uniformity of less than 3 ppm required in the DSV. Achieving this uniformity, however, requires an extremely high outlay in the design of the basic field magnet, which is composed of a plurality of superconducting coils. Typically, six coils are required in order to achieve the uniformity cited above as an example.
However, European Application 0 399 789 discloses a magnetic resonance tomography apparatus that makes use of inhomogeneities of the basic magnetic field for imaging purposes. Gradients of the basic magnetic field on the order of 0.1 T/cm are thereby generated, these being superimposed with switched gradient fields having values of, typically, 0.2 T/cm.
German OS 33 04 461 discloses a magnetic resonance tomography apparatus wherein the homogeneity range of the basic magnetic field is less than the measuring field. The gradient field, however, proceeds monotonously over the entire measuring field, preferably increasing/decreasing linearly. A nuclear magnetic resonance tomogram thus exhibits less resolution in the edge region than in the central, diagnostically interpretable region.